1. Field of the Invention
The present invention relates to an image processing apparatus, and particularly to an image processing apparatus for synthesizing a plurality of image data items to create one image.
2. Description of the Related Art
Lossy compression of, for example, a JPG file can create a small file with extremely high compression rate. Such compression is effective for a picture, but does not have good readability of characters in a binary image such as a character image because an edge of a character blurs.
Therefore, as disclosed in Japanese Patent Application Laid-Open No. 2002-368986 (Prior Art 1) and Japanese Patent Application No. 3193086 (Prior Art 2), there is proposed a technology which is capable of creating an image file having high compression rate, while maintaining high resolution of a edge of a character by outputting the character at binary resolution, by selectively switching two image data items, i.e. image data with high compression rate and binary data with high resolution in lossy compression, which are contained in one file, with binary data (character region) having high resolution.
When a user uses such a file created in the above method, a plurality of compressed image data items are expanded, and synthetic image data items are displayed by an application on a monitor, or the expanded and synthesized image data items are printed out by a printer or the like.
However, when performing such synthesis, there arise various problems attributable to the differences in resolution. For example, Japanese Patent Application Laid-Open No. H11-261833 (Prior Art 3) and Japanese Patent Application Laid-Open No. 2003-219187 (Prior Art 4) describe these problems as follows.
Prior Art 3 mentions a problem caused when synthesizing an image having data selecting a character part and non-character part, and an image containing a picture having resolution different from that of the above data. Specifically, the image containing a picture has pixels which are selected and pixels which are not selected, and the pixels which are not selected may have any pixel values. The pixels which are selected are called “effective pixels”, and the pixels which are not selected are called “ineffective pixels”. When the image containing a picture is filed by reducing the resolution thereof lower than that of selected data, the image containing a picture is enlarged at the time of synthesis. When the image containing a picture is enlarged, an interpolation algorithm such as a bilinear method is often used. When enlarging the image, a border section between the effective pixels and ineffective pixels becomes an intermediate value between the effective pixels and ineffective pixels after enlargement. As a result, blurring occurs in the border section. In order to avoid such blurring, the pixel values of the ineffective pixels are prevented from being affected even when increasing the effective pixels to take an intermediate value.
Prior Art 4 discloses how one image data item is expressed by three image data items. One of the three image data items is obtained by performing reduction processing (pixel density conversion) on an image which is obtained by binarizing an original image. Prior Art 4 also discloses positional displacement caused at the time of reduction processing, in the difference in resolution between the original image and the image data obtained after the reduction processing.
When displaying/printing a synthetic image, if the resolution of the image data configuring the synthetic image is different, displacement of the positions of pixels between the images occurs in the synthetic image, causing visual distortion of the character part and color shift. How such displacement occurs depends on the algorithm for enlarging image data. Such a problem occurs under conditions where there is a remainder when the number of vertical and horizontal pixels are divided by a common multiple of the ratio of the resolution of each image data item in vertical and horizontal directions. When the remainder is 0, this problem does not occur.
Also, the above-described conventional technologies for generating a synthetic image do not mention the above problem. The technology disclosed in Prior Art 3 is for avoiding blurring created in the border section by the interpolation algorithm as described above, and thus does not at all describe the technical means for solving such problem. Moreover, Prior Art 4 mentions the displacement caused by the difference in resolution between the original image and the image data obtained after the reduction processing as described above, but does not at all describe problems caused by the difference in resolution between the constituent image data items.